Phono preamplifiers incorporating the RIAA network typically utilize a 47k ohm resistor, electrically connected in parallel with the preamplifier input, to present a controlled, predetermined load impedance to the magnetic phono cartridge. Some recent equipment utilizes switch-selectable resistors, typically in the range 10k ohm to 100k ohm, and also capacitive loading in the range of a few hundred picofarads, which may also be switch-selectable. These approaches represent an attempt to modify high-frequency response characteristics of the phono cartridge-preamplifier system. Many magnetic cartridges exhibit mechanical resonances which produce a peaking of the overall response characteristic at one or more frequencies in the high-audio frequency range.
The voltage output of the magnetic cartridge is dependent upon the electrical load impedance to which it is connected. In the high-audio frequency range, the output impedance of the magnetic cartridge appears substantially inductive. Thus, the capacitance of the cable which connects the magnetic phono cartridge to the preamplifier forms a parallel resonant circuit with the intrinsic inductance of the cartridge, resulting in response maximum at the resonant frequency. The canonical 47k ohm load suppresses the Q of this resonance and the combination produces a reasonably flat response over the audio range of frequencies, as can be seen from FIG. 11. If a load resistance greater than 47k ohm (e.g., 1M ohm) the mid-band output voltage is increased at the expense of frequency response. A lower impedance resistive load (i.e., lower than 47k ohm) produces even greater bandwidth, but output voltage amplitude is sacrificed. In general, then, a relatively low-resistance load can be seen to produce best frequency response, but with low output voltage amplitude and hence poor signal-to-noise.
Because of their lower moving mass, the newer moving coil type cartridges appear to have less tendency to color the sound by mechanical resonance. However, some manufacturers and audiophiles have suggested operating these relatively low impedance cartridges into load resistances of the order of 20 ohms to 100 ohms, and in some cases as low as 2 ohms, to modify the "color" of the sound.
Both the low impedance moving coil type and the high impedance moving iron or moving magnet type cartridges, by virtue of their non-zero output impedances, develop an output voltage which is related to the complex load impedance, as well as to the electrical and mechanical characteristics of the cartridge itself (and the program material, of course). The prior-art approaches to preamplifiers for these types of phono cartridges have generally utilized passive impedances (e.g., resistors, capacitors, inductors) in various degrees of compromise between the twin ideals of wideband flat frequency response, on the one hand, and low noise, on the other.
Typical of prior-art patents in related areas is U.S. Pat. No. 4,134,140, "Voltage Mode Amplifier for Use with a High Q Magnetic Head," issued to C. E. Wright et al. an Jan. 9, 1979, which discloses the use of shunt feedback in connection with a magnetic tape playback head. FIGS. 1A and 2 of this reference show the shunt feedback arrangement, and FIG. 1B illustrates the effect on the resonant peaking of the magnetic pickup head. The discussion at column 1, lines 16-36 provides a description of the design considerations involving in a preamplifier for a magnetic tape playback head.
U.S. Pat. No. 4,105,945, "Active Load Circuits," issued to N. Sano et al. on Aug. 8, 1978, describes the use of an "active load" (the components designated 13-18 in FIG. 2 of the reference) to produce, by means of negative feedback, an input impedance that is lower than the intrinsic input impedance of the equalizer amplifier. This reference does not appear to teach the advantages to be derived from a simple virtual-ground approach.
U.S. Pat. No. 4,032,855, "Phonograph Preamplifier Network with Infrasonic Cutoff Response," issued to T. M. Holman, II, on June 28, 1977, describes the use of a hybrid feedback scheme which is part series feedback, part shunt feedback. FIG. 1 and the description commencing at column 2, line 47, indicate that this invention is directed toward modifying the very low frequency ("infrasonic") response of the preamplifier.
U.S. Pat. No. 4,117,412, "Phonograph Preamplifier Network with Infrasonic Cutoff Response," issued to T. M. Holman, II, on Sept. 26, 1978, is a continuation-in-part of the same application as matured into U.S. Pat. No. 4,032,855. The claims are somewhat different, although there appears to be no additional disclosure that might be relevant to the present invention.
U.S. Pat. No. 4,210,942, "Video Signal PlayBack Circuit," issued to H. Nakamura et al. on July 1, 1980, illustrates a similar circuit arrangement as applied to the playback head of a video magnetic tape apparatus. In the schematic diagram of FIG. 2, resistor 19 appears to provide shunt feedback. Especially relevant is the discussion of signal to noise at column 1, line 62, through column 2, line 31.
U.S. Pat. No. 4,041,158, "Low Noise Magnetic Transducer Preamplifier Having Flat Response," issued to R. R. Parker on Aug. 9, 1977, is of general interest only.
Some of the considerations influencing design of low noise amplifiers generally are summarized in the article by H. P. Walker, "Low-noise Audio Amplifiers," Wireless World, May 1972 (pp. 233-237).
Also if interest is the article by T. Holman, "New Factors in Phonograph Preamplifier Design," Journal of the Audio Engineering Society, May 1972 (pp. 263-270).